Hybrid-earth-station (HES) systems include receivers capable of receiving and demodulating satellite signals modulated with both voice and data information. Typically, an HES receiving site includes a satellite dish coupled to several signal downconverter stages. The satellite dish is oriented toward the path of the satellite and receives a broadband transmission of Ku-band signals containing voice and data information. A downconverter stage on the satellite dish shifts the Ku-band signals down to L-band. The L-band signals are then transmitted along a cable to an L-band radio where another downconverter stage further shifts the frequency of the signals and retrieves the information carried on the signals.
In each instance of shifting, the incoming signals are shifted from one predetermined frequency bandwidth to another. One or more carrier frequencies, each modulated to carry voice or data information, may exist in the shifted bandwidth. Demodulators in the L-band radio each look for a particular carrier frequency in the shifted bandwidth to demodulate.
The downconverters often shift a band of frequencies more or less than desired. Such shifting inaccuracies can be caused by temperature changes in the oscillators located in the downconverters. Inaccurate shifting results in the signals arriving at the demodulators offset from the expected frequencies.
Voice information is particularly sensitive to any frequency offsets that originate in the HES receiver or at any other point prior to retrieving the information from the signal. Previously, receivers attempted to combat frequency offsets by using very accurate, but expensive, oscillators and frequency reference devices to ensure accurate frequency shifting.
Another approach to combatting frequency offsets is to use demodulators that have broad searching and tracking capability to individually search the frequency band for the particular carrier the demodulator has been assigned. However, the typical demodulator can only sweep small segments of a frequency spectrum at one time. After sweeping each small segment, the demodulator must jump to the next segment and again sweep for its assigned carrier frequency. An audible "click" occurs on a telephone communication line each time the demodulator jumps to another segment.
Accordingly, there is a need for an accurate receiver that is inexpensive and can adequately compensate for the frequency offsets that can occur in voice communications.